The present invention pertains to decoders used in communication systems.
Encoders and decoders are used in communication systems in order to, among other things, increase the robustness of transmission of information through a noisy channel. The cascading or concatenation of error control codes is a well known technique in digital communications. This type of code can yield excellent bit error rate performance. Concatenated codes that contain short memory convolution codes are applicable to many communication links. The applications include various combinations of modulations with memory, channels with memory and coding with memory. The Maximum Likelihood decoder (MLD) is the decoder of choice for these concatenated coding schemes. A Viterbi decoder is an example of an MLD decoder. Unfortunately, prior art MLDs produce only hard decisions. The MLDs near the channel (inner decoders) therefore do not send all the available symbol information (soft decisions) to the outer decoders. Also, there are no practical decoders that produce this symbol information. The result is an unrealized coding gain of 2 dB or more depending upon the channel type.